Women with a known prior blood clot during pregnancy, artificial heart valves or other genetic conditions are at a very high risk for these complications during their pregnancy. It has been well established that these women benefit from medical treatment with a blood thinning medication in their pregnancies to prevent further formation of blood clots. These medications are called Heparins and are given as shots. Prior studies have suggested that a type of Heparin called "low molecular weight heparin" (Enoxaparin=Lovenox®) is well suited for use in pregnancy as it does not affect the baby and has a very low complication rate.